Pulse multiplex transmitter employing a cathode-ray tube time modulator



1948- P. K. CHATTERJEA ETAL 2,454,773

' PULSE MULTIPLEX TRANSMITTER EIPLOYING A CATHODE'RAY TUBE TIME MODULATOR Original Filed March 14, 1944 Em J'ElliRATE 31mm. fol/Regs EL 5b 7320 my HUYFULLH Kumnk, (nan-ma I-EsuE MLFRQD. HoucnToN Patented Nov. 30, 1948 PULSE MULTIPLEX TRANSMITTER EM- PLOYING A CATHODE-RAY TUBE TIME MODULATOR Prafiilla Kumar Chatterjea and Leslie Wilfred Houghton,

London, England, assignors to Standard Telephones and Cables Limited, London, England, a British company Original application March 14, 1944, Serial No.

526,566. Divided and this application October 6, 1945, Serial No. 620,818. In Great Britain April 7,1943

The present invention nal high frequency transmission systems of the kind utilising electrical pulses. This application is a division of our co-pending application Serial No. 526,566, filed March 14, 1944, now Patent No. 2,447,233, granted August 1'7, 194

It is now well known that signal Waves bearing intelligence can be transmitted by means of a train of electrical pulses occurring at equal time intervals and whose durations are varied in accordance with the instantaneous amplitude of the signal. Such pulses are herein referred to as duration modulated pulses. In a modification of this system of signal wave, transmission, short electrical pulses of constant duration are transmitted to mark the leading and trailing edges of duration modulation pulses, and when either the leading or trailing edge occurs at equal intervals of time, only the pulse marking the edges of the duration modulated pulses which occur at varying intervals of time need be transmitted. This type of transmission is sometimes referred to as a single pulse or time modulated or time-phase pulse transmission or a pulsed phase modulation, the pulses of constant short duration occurring at varying intervals of time in accordance with the instantaneous amplitude of the signal wave to be transmitted.

When both the fixed and moving marking pulses of a duration modulated system are transmitted, the system is referred to as a double pulse system. In the single pulse system, the fixed pulse may be reinserted at the receiver and the solid pulses or duration modulated pulses obtained from the fixed and movable pairs of pulses for example by means of known build back circuits. If in the single pulse system the pulse duration is only a small fraction of the time interval between the occurrence of successive pulses, and if the variation of this time interval taken up for a maximum time or phase modulation of the pulses is also small, then other trains of similar pulses may be transmitted having the same pulse repetition frequency, but whose pulses occur in'the intervals between the pulses of another train. Thus each pulse train may bear time or phase modulation of its respective signal wave and a multiplex channel transmission system is provided. It is the objectof this invention to provide practical arrangements whereby such a system may be put into practice.

In systems of this type four main operations are essential. Firstly, producing the trains of unmodulated or unphased pulses, one train for each channel, the" pulses of'each train being correlates to multiplex sig-= 5 Claims. (01. 179-15) 2 rectly timed or phased or time-phased to occur with respect to the pulses of other trains of the system, and modulating the pulses of each channel with the respective signal Wave. Secondly combining all the channels into one transmission. Thirdly, at the receiving end of the system directing the pulses of a channel to respective receiving apparatus. Fourthly, obtaining the signal waves from the train of pulses in the respectivechannel.

It will be understood that more than one of these operations may be performed by the same equipment that the pulses may be transmitted by line, wave guide or radio, and may comprise direct current pulses or trains of waves of higher frequency than the pulse repetition frequency.

According to the present invention arrangements for producing the trains of pulses of the channels in a multi-channel electrical pulse signalling system of the kind in which the pulses of a channel are time modulated in accordance with the signal wave to be transmitted by that channel, comprises a target for each channel, said target being suitably shaped for the type of time modulation desired, means for deflect ing an electron beam periodically across the several targets at successive instants under the con trol of a wave of constant form to set up a series of sharp edged pulses for each channel and means for deflecting the electron beam passing across a traget transversely of the first deflection under the control of the signal Wave of the channel specified by said target, and means for combining all the pulses from the several targets into a single pulse train. I I

Suitable targets for producing either duration modulated pulses or time-phased modulated pulses are described in British patent specification No. 523,575.

In carrying out the invention a cathode ray tube having a target suitably shaped for the type of time modulation desired, located at the end of the tube opposite the electron gun is provided for each channel. The electron beam is deflected across the target in one direction by a wave such as a sawtooth waveform, the sawtooth waveforms from the same source applied to the several cathode ray tubes being suitably phased with respect to each other so that the beam sweeps across the targets takes place at successive instants. The electron beam is deflected in a perpendicularly related direction to the first mentioned direction, i. e., longitudinally of the target under the control of the signal wave. A resistance is provided in'series with the target from across which resistance the time modulated pulses are obtained. This resistance may be common to all the targets.

In another Way of carrying out the invention a single cathode ray tube is provided for all the channels, and is provided at the end of the tube with a plurality of radially disposed targets, of suitable shape depending upon the type of time modulation desired, there being one target for each channel. There are also provided. means for producing a rotation of the electron beam at the pulse repetition frequency, and a plurality of deflecting means, one for each channel, for producing a radial deflection of the beam, and means for applying the signal waves of the various channels to respective ones of said deflecting means. The targets have a common terminalconnected to earth potential via a resistance across which the time modulated pulses of the channels are obtained.

When time phase modulated pulses are desired said targets may comprise narrow bars and when duration modulated pulses are desired said target.

may comprise triangular plates. It will be clear to those versed in the art that the metallic bars or plates may be replaced by correspondingly shaped apertures in a plate which impedes the electron stream, this latter only reacting the collecting electrode through the apertures. The pulses are then obtained from a resistance connected between the collecting electrode and earth potential. In the accompanying claims the word target includes such a construction.

The invention will now be described with reference to the drawings accompanying the provisional specification, in which. Figure 1 shows diagrammatically a cathode ray tube as disclosed in the aforementioned British patent specification No. 523,575 individual to each channel, and. Figure 2 shows diagrammatically the case of a.

cathode ray tube having a plurality of target bars.

In the arrangement indicated in Figure 1 a cathode ray tube for each channel has two pairs of perpendicularly related deflecting plates or coils XX and YY but instead of the usual screen a metal bar indicated at B is mounted at the end of the tube. By the application of an A. 0. voltage to one pair of deflecting plates XX the electron beam is made to scan across as shown by arrow H at a frequency corresponding to the mean recurrence frequency of the required pulse train. It will be seen that the beam can be madev to strike the bar B once ever cycle by blocking out the return stroke, and so doing will produce a. voltage pulse across resistance R at terminal T. By applying the modulation signal voltage tothe other pair of deflector plates YY the scanning line H is made to move at right angles to itself as indicated by the arrow V, so causing the beam to hit the plate I at a point depending on the beam position. Hence the instants of occurrence of pulses at T will depend on the signal modulation voltage. Although deflecting plates have been referred to it will be understood that deflecting currents may be used instead of deflecting voltages, in which case current carrying coils would be used instead of the plates XX, YY.

The horizontal deflecting A. C. voltage is preferably of sawtooth waveform and the sawtooth waveforms for the respective channels ar relatively phased with respect to each other, the wave form efiecting the scanning of, each tube only during a portion of the waveform cycle which has the pulse repetition. frequency. Each, bar B may be connected through the same resistance R to earth potential in which case all the channel pulses willbe obtained from T.

In the arrangement shown in Figure 2 a circular scan is employed instead of the scan across the tube, and a set of deflector plates and a target bar are provided for each channel. The deflector plate assembly consists of say a common plate. I and for example four plates 2, 3, 4 and 5 between each of which and the common plate I are applied the. corresponding channel modulation signals. The target may consist of one structure providing the correctly placed bars on which are produced the required phased pulses which are obtained at. say, terminal 6, from the endv of. resistance. R. The target bars a, b, c, d are connected at one end to a common terminal plate e which in turn is connected to earth potential through resistance R.

The beam is caused torotate at the pulse repetition frequency by known means, for example, two voltages at the correct frequency in quadrature, phase applied between perpendicularly related pairs of deflecting plates. The, unmodulated trace of the beam is shown at ,f in Fig. 2, and when the modulating potentials are applied to the electrodes [-2, l-3, I4. and l-5, the radial deflection of the beam is varied thus varying the times at which itv strikes the bars. a, b, c. and d and produces corresponding time-phased! pulses in the resistance R.

What is claimed is:

1.. In a multi-channel. pulse modulator, means. to. producev a beam of electrons, means to cause. the beam to have a given sweep movement, a plurality of signal channel input circuits, a plurality of deflection means each associated with av separate one or said input circuits, to control deflection of the beam successively during its. sweep movement according to increments of the input signals of said circuits, and beam responsive means to produce pulses of energy time modulated according to the corresponding signal deflections of said beam.

2. A modulator according to claim 1, wherein the. means for controlling deflection ofv the beam includes beam deflecting electrodes, and means for connecting said circuits each to a separate beam deflecting electrode.

3. A multi-channel pulse modulator comprising means to produce a beam. of electrons, beam deflection means, a source of control wave energy, means. for. applying said control wave energy to said deflecting means to cause. they beam to have a given sweep movement, a plurality of signal channel input circuits, a plurality of deflection means each associated with a separate one of said input circuits, to control deflection of the. beamsuccessively during its sweep movement ac.- cording to increments of the input signals of said circuits, and beam responsive means presenting narrow target, areas for impingementv by electrans of said beam, one for eachv signal channel, said target areas being disposed at an acute angle to the direction. of signal deflection of said beam for the corresponding signal channel to produce pulses of energy modulated according to the corresponding signal deflections, of said beam.

4. A modulator according tov claim 3, wherein the means for controlling signal deflection of the. beam includes a. circular electrode disposed centrally of the, sweep path of said beam and aplurality of small electrodes disposed. in, sequential relation along thesweep path oi said. beam and 5 spaced from said circular electrode to provide passage of the beam therebetween, and means for connecting said circuits to each one of said small electrodes.

5. In a plural channel transmitter, a modulating device comprising a cathode ray tube having means for producing an electron beam, a target electrode, means causing the beam to be deflected through a plurality of positions corresponding to channels and to impinge on the target at a distinct reference time point for each channel, deflection means individual to said positions and responsive to a signal voltage to cause said beam to impinge said target at a point time-displaced from said reference point an amount corresponding to said signal voltage, and translating means to transmit the target impingements into a series of pulses in coordinated time relation.

PRAFULLA KUMAR CHAT'I'ERJEA. LESLIE WILFRED HOUGHTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,199,634 Koch May 7, 1940 2,256,336 Beatty Sept. 16, 1941 2,263,369 Skillman Nov. 18, 1941 2,265,216 Wolf Dec. 9, 1941 2,265,337 Beatty Dec. 9, 1941 2,298,922 Beatty Oct. 13, 1942 2,328,944 Beatty Sept. 7, 1943 

